Suppression of single versus multiple salient distractors in visual search displays

While salient objects tend to “pop-out” from a visual scene, capturing our attention, we can also learn to suppress them when task-irrelevant. Distractor suppression effects associated with salient information have been demonstrated behaviourally and with event-related potentials (Pd components). Research on distractor processing has focused on single-item suppression, and it remains unknown whether suppression can be applied to multiple distractors in the same search display, either in parallel or sequentially. We tested this using displays that contained a highly salient colour distractor (S+) and/or a less salient colour distractor (S-). Behavioural and electrophysiological data were collected. Lateralised activity associated with S+ and S- was measured separately by examining displays where one singleton appeared laterally while the other singleton (and the target) appeared vertically. Behaviorally, task performance improved with the number of salient distractors (0, 1, or 2), suggesting that suppression can be applied to multiple objects. Moreover, the Pd component was enhanced for both S+ and S- when the number of distractors increased from the previous trial, providing new electrophysiological evidence for intertrial priming of multiple distractor suppression. However, while Pd components were elicited by S+ regardless of whether S- was present, S- triggered a Pd when it was the only salient distractor but not when S+ was present. This suggests that Pd components are only triggered by the most salient display item. Thus, while behavioural evidence supports the notion that multiple salient distractors can be suppressed, electrophysiological results are mixed. Intertrial priming effects suggest that distractor numerosity affects suppression in the subsequent display, but there was no evidence of S- suppression when presented alongside S+. Our results suggest that single-distractor and multiple-distractor suppression may involve different mechanisms. They also show that the relative saliency of distractors determines their ability to trigger Pd components.